Optical communication transmission system

ABSTRACT

An optical communication transmission system including an optical amplifier lumped repeater system of the present invention includes, for the purpose of preventing degradation of the transmission characteristic arising from wavelength dispersion of optical fibers due to raised power of the optical signal, transmission optical fibers provided for all or most of the repeating sections and having wavelength dispersion values set to different values from zero, and optical fibers provided for the individual sections to compensate for the sum of wavelength dispersion of the sections so as to reduce the total wavelength dispersion to zero. The optical fiber for compensation for each section may be replaced by a substitutive compensation element. Alternatively, very small wavelength dispersion which remains due to failure in compensating to zero dispersion may be compensated for using a dispersion equalizer of an electric system in the reception section.

This is a Continuation of application Ser. No. 08/079,554 filed Jun. 22,1993 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high-speed, long-haul communicationtransmission circuit by an optical fiber, and more particularly to anoptical communication transmission system which is expected to bedeveloped as a communication system for a transmission network foradvanced information service and which can transmit a large amount ofinformation with a high degree of quality over a long distance.

2. Description of the Related Art

An optical communication transmission system makes use of the broad bandfeasibility of light to permit high-speed, very high-capacity,high-quality communications which cannot be realized readily withconventional communications using the microwave band or the millimeterwave band. For example, the following reports have been provided withregard to elements for use with communication of, for example, 10Gbit/s:

by T. Suzaki et al., “10-Gbit/s Optical Transmitter Model withMultiquantum Well DFB LD and Doped-channel Hetero-MISFET Driver IC,”1990 Optical Fiber Communication Conference, Technical Digest TUI2, and

by T. Suzaki et al., “Ten-Gbit/s Optical Transmitter Module UsingModulator Driver IC and Semiconductor Modulator,” Optical FiberCommunication Conference 1992, Technical digest TUI6.

An optical communication transmission system of the optical amplifierlumped repeater system which uses erbium-doped optical fiber amplifierswill be described with reference to FIG. 1.

An optical transmitter 3 modulates optical power outputted from asemiconductor laser source 1 by intensity modulation by an externalmodulator 2 of lithium niobate LiNbO3 which is driven by a signal of 10Gbit/s outputted from a modulation signal source 5 and outputs themodulated optical power to an optical power amplifier 11. The opticalpower amplifier 11 consists of an erbium-doped optical fiber amplifierand amplifies a signal light level and outputs the amplified opticalsignal to a first optical fiber 101 for a transmission line of anoptical amplifier lumped repeater system. In this instance, when thesignal light level exceeds 10 dBm, in order to avoid the influence ofBrillouin scattering in the transmission fiber, the line width of thesemiconductor laser is expanded in advance using the well-knowntechnique of direct FM modulation of the semiconductor laser or a liketechnique. After passing the optical fiber 101, the optical signal isamplified again by a direct optical amplifier repeater 12 which consistsof an erbium-doped optical fiber amplifier and is then outputted to asecond stage optical fiber 111 for transmission. The signal lightinputted into the transmission line at the second stage is amplified bya second stage optical amplifier repeater 13 and outputted to a thirdtransmission line 121. The signal light is thereafter processed in asimilar manner and transmitted finally to a last transmission line 191.In an optical receiver 53 on the reception side, the optical signal isamplified by an optical preamplifier 21 and converted into an electricsignal using a PIN photodiode 51, which is a photoelectric transducer.The electric signal, and consequently, the signal of 10 Gbit/stransmitted from the modulation signal source 5, is then reproduced byan equalizer amplifier regeneration circuit 52.

In the high-speed, high-capacity communication system described above,however, it is known that waveform distortion after transmission due tosuch causes as chromatic dispersion of the optical fibers stronglydegrades the transmission characteristic through a very long distancetransmission.

Therefore, the following countermeasures are conventionally taken:

First, as a countermeasure to chromatic dispersion of an optical fiber,which is conventionally considered to be the most significant cause ofdegradation of the transmission characteristic, a transmission line isconstructed using an optical fiber which has no chromatic dispersion inthe waveband of the light source of the optical transmitter. In otherwords, the optical fiber employed has zero chromatic dispersion.

For example, as a communication system for a long-distance submarinecable, transmission systems wherein the dispersion value of an opticalfiber for transmission is reduced substantially to zero have beenproposed by:

N. S. Bergano et al., “9000 km, 5 Gbit/s NRZ Transmission ExperimentUsing 274 Erbium-doped Fiber-Amplifiers,” Technical Digest of TopicalMeeting on Optical Amplifiers and Their Applications, Santa Fe, Jun.24-26, 1992, postdeadline paper PD11, and

T. Imai et al., “Over 10,000 km Straight Line Transmission SystemExperiment at 2.5 Gbit/s Using In-Line Optical Amplifiers,” TechnicalDigest of Topical Meeting on Optical Amplifiers and their Applications,Santa Fe, Jun. 24-26, 1992, postdeadline paper, PDI2.

In an actual transmission line, however, the requirement for zerochromatic dispersion cannot be fully satisfied over the entire length ofthe optical fiber, and very small level of chromatic dispersion exists.In order to suppress the influence of the very small dispersion, severaltechniques for compensating for the chromatic dispersion in thetransmitter side and the receiver side have been proposed, for example,in Japanese Patent Laid-open No. 1987-65529 and Japanese PatentLaid-open No. 1987-65530, and by:

A. H. Gnauck et al., “Optical Equalization of Fiber Chromatic Dispersionin a 5 Gbit/s Transmission System,” Optical Communication Conference,San Francisco, Jan. 22-26, 1990, postdeadline paper PD7, and

N. Henmi et al., “A Novel Dispersion Compensation Technique forMultigiga-bit Transmission with Normal Optical Fiber at 1.5 MicronWavelength,” Optical Fiber Communication Conference 1990, postdeadlinepaper PD8.

Further, in a coherent communication system, such techniques asequalizing an electric signal in the receiver side by using a delayequalizer at the stage of an intermediate frequency of the electricsignal have been reported by:

K. Iwashita et al., “Chromatic Dispersion Compensation in CoherentOptical Communications”, IEEE, Journal of Lightwave Technology, Vol. 8,NO. 3, March 1990, pp. 367-375.

It is known that the causes for degradation of the transmissioncharacteristic of an optical amplifier lumped repeater system include,in addition to wavelength dispersion of the optical fiber describedabove, a noise accumulation effect caused by spontaneous emission lightand a noise increase effect caused by a non-linear effect in the opticalfiber through multistage optical amplifier repeaters. In order todecrease the influence of the accumulation effect of noise ofspontaneous emission light, the outputs of the optical amplifierrepeaters must be set high. On the other hand, in order to suppress thenon-linear effect in the optical fiber, the outputs of the opticalamplifier repeaters must necessarily be set low. Due to these twocontradictory requirements, it is conventionally difficult tosimultaneously control both the noise accumulation effect and thenon-linear effect. Therefore, in order to obtain a very long-haultransmission system or achieve an increase of the repeating distance, itis necessary to increase the repeater output while decreasing thenon-linear effect in the optical fiber.

However, little is known of the non-linear effect in an optical fiber,and the causes of degradation have not been specifically identified asyet.

SUMMARY OF THE INVENTION

It is believed that a self-phase modulation effect is a major factor inthe non-linear effect in an optical fiber. However, as recently reportedby S. Saito et al. [“2.5 Gbit/s, 80-100 km Spaced In-line AmplifierTransmission Experiments Over 2,500-4,500 km,” Technical Digest ofEuropean Conference on Optical Communication 1991, postdeadline paper3], in addition to the self-phase modulation effect, noise is increasedby the influence of a 4 wave-mixing effect between signal light andspontaneous emission light outputted from the optical amplifier,resulting in the degradation of the transmission characteristic.

Further, in addition to the self phase modulation effect, a noiseincrease believed to arise from a non-linear effect in an optical fiberfor each section of a multistage optical amplifier lumped repeatersystem was discovered in experiments conducted by the inventors of thepresent application which will be hereinafter described.

It has been made clear that those noise-increasing effects, other thanthe self-phase modulation effect, increase with the increase of thesignal power and the increase of the transmission distance, and noise isproduced over the full length of the transmission line, resulting in agreater spectrum spread and a greater degradation of the signal-to-noiseratio than the self-phase modulation effect. Accordingly, it has becomeclear that the transmission limit is restricted by the non-linear effectin the optical fiber.

It has become apparent through experiments that the non-liner effect inthe optical fiber occurs when the transmission light power is high butis deterred when the optical fiber for transmission does not have a zerodispersion wavelength at the wavelength of the optical signal.Therefore, if an optical fiber which does not have a zero dispersionwavelength at the wavelength of the optical signal is employed as theoptical fiber for transmission, the non-linear effect in the opticalfiber can be suppressed even when the transmission light power is high.

It is an object of the present invention to provide an opticalcommunication transmission system including an optical amplifier lumpedrepeater system wherein very high-speed, high-capacity and long-hauloptical communications can be realized with a high degree of quality.

In order to attain the object described above, an optical communicationtransmission system of the present invention includes transmissionoptical fiber means having a zero dispersion wavelength of a valuedifferent from the transmission wavelength of the optical transmittermeans with at least two connections between the optical transmittermeans and the optical receiver means, and dispersion compensation meansfor making the sum total of wavelength dispersion substantially equal tozero when the sections are arranged in cascade connection.

In an embodiment of the present invention, the dispersion compensationmeans is included in each of the sections of the transmission opticalfiber means or in the optical transmitter means or the optical receivermeans. Further, an optical signal is modulated by the opticaltransmitter means and received in a coherent system by the opticalreceiver means, and the influence of wavelength dispersion upon theoptical signal over the entire transmission line is compensated by theelectric dispersion equalization means. The type of modulation by theoptical transmission may be optical frequency modulation, phasemodulation or polarization modulation.

Further, the present invention can be applied readily to a conventionalsystem by installing a dispersion compensation optical fiber for atransmission optical fiber, which is conventionally provided on theoutside, inside an optical repeater and replacing the optical repeater.Alternatively, it is possible to install a small dispersion compensatorsuch as a grating pair in the apparatus in place of the dispersioncompensation optical fiber.

In summary, according to the present invention, in order to suppress thenon-linear effect in an optical fiber, the zero dispersion wavelength ofthe transmission optical fibers, which is conventionally made tocoincide with the transmission wavelength, is shifted from thetransmission wavelength for each section. By virtue of this means, thepresent invention has the advantage that the transmission optical powerof an optical amplifier lumped repeater system can be increased so as toimprove the transmission characteristic, and consequently, a veryhigh-speed, very long-haul optical communication transmission system canbe realized readily.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description referringto the accompanying drawings which illustrate the examples of thepreferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a conventional optical amplifier lumpedrepeater system;

FIG. 2 is a spectrum diagram of the received signal when, in therepeater system of FIG. 1, the wavelength of the semiconductor lasersource is set to the conventional standard of 1.552 μm and the power ofthe transmitter optical signal is raised;

FIG. 3 is a spectrum diagram of the received signal when the wavelengthof the semiconductor laser source is set to 1.547 μm and the power ofthe transmission optical signal is raised;

FIG. 4 is a spectrum diagram of the received signal when the wavelengthof the semiconductor laser source is set to 1.557 μm and the power ofthe transmission optical signal is raised;

FIG. 5 is a diagrammatic view of an optical amplifier lumped repeatersystem of a first embodiment of the present invention;

FIG. 6 is a diagrammatic view of an optical amplifier lumped repeatersystem of a third embodiment of the present invention; and

FIG. 7 is a diagram illustrating a code error ratio characteristic whenthe present invention is used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The results of experiments with the conventional optical communicationtransmission system shown in FIG. 1 will first be described forcomparison with the present invention.

In the transmission system of FIG. 1, the wavelength of thesemiconductor laser source was set to 1.552 μm and a 1,000-kmtransmission experiment using a stage optical amplifier repeater wasconducted using, for the optical fibers 101, 111, 121, 131, . . . and191, a dispersion shifted fiber of 100 km whose zero dispersionwavelength was 1.552 μm. Here, the transmission loss of the dispersionshifted fiber per 100 km was 22 to 23 dB, and the noise figures of theoptical power amplifier 11, the optical amplifier repeaters 12, 13, 14,15, . . . and 20 and the optical preamplifier 21 were 8 to 9 dB. Whenthe output levels of the optical power amplifier 11 and the opticalamplifier repeaters 12 to 20 were set to approximately 1 dBm, the powerlevels of the input signal light power into the optical amplifiersdropped to −21 to −22 dBm, and consequently, the reception failed due tonoise increase by noise accumulation of the spontaneous emission line.Thus, the optical amplifier repeater output power level was increased,but a good transmission characteristic was not obtained even when thesignal level was raised to +11 to +12 dBm. FIG. 2 shows the receptionsignal spectra. It can be seen that when the transmission signal lightlevel is raised, the signal-to-noise ratio of the signal light level isdegraded conversely due to a non-linear effect in the optical fiber.

As a preliminary experiment of the present invention, the sameexperiment was conducted with the same transmission system as that ofFIG. 1 changing the 1.552 μm wavelength of the semiconductor lasersource to 1.547 μm and 1.557 μm. Here, the dispersion shifted fibers101, 111, 121, . . . and 191 had the dispersion values of D=−0.35ps/km/nm and D=+0.35 ps/km/nm, respectively, for the two wavelengths.Observation of the reception spectra after transmission line revealedthat, as can be seen in FIGS. 3 and 4, the signal-to-noise ratio aftertransmission is improved at each of the anomalous (D>0) and normal (D<0)dispersion values. However, since the amount of dispersion of the entiretransmission line was great, the waveform distortion after transmissionwas too great to receive the signal.

Conventionally, it is believed that the transmission characteristicdegradation by a non-linear effect in an optical fiber arises fromwaveform distortion by self-phase modulation, but according to theexperiments, a noise-increasing effect due to the non-linearity in theoptical fiber has been observed.

While the cause of the noise-increasing effect is unknown, the inventorshave clearly shown, based on the experiments, that the noise increase isgreat when the signal light has the same wavelength as the zerodispersion wavelength in the optical fiber but is small when the signallight does not have the same wavelength as the zero dispersionwavelength in the optical fiber. Also it has been observed that as thetransmission distance increases, the noise component also increases, andit has been found out that the noise is produced over the entire lengthof the optical fibers constituting the transmission line and suppressionof the noise increase is significant in the normal (D<0) dispersionregion.

The first embodiment of the present invention will next be describedwith reference to FIG. 5.

The wavelength of a semiconductor laser source 1 is set to 1.547 μm, andoptical fibers 101, 111, 121, 131, . . . and 191 of a transmission lineare constituted from dispersion shifted fibers whose zero dispersionwavelength is 1.552 μm. Conventional fibers 102, 112, 122, 132, . . .and 192 which have anomalous dispersion (D>0) are inserted after thedispersion shifted fibers 101 to 191 of the individual transmissionsections for compensating for the wavelength dispersion of therespective fibers 101 to 191. Since the amount of dispersion of thedispersion shifted fiber for each section is −35 ps/nm per 100 km, theconventional (D>0) fibers of about 2 km (dispersion value 35 ps/nm) werearranged in cascade connection to set the total amount dispersion ofeach section to a value in the proximity of 0 ps/nm. As a result, whenthe repeater output was higher than +8 dBm, a good transmissioncharacteristic was obtained wherein the reception sensitivitydegradation after transmission was approximately 1 dB.

Further, as a second embodiment, in place of the conventional (D>0)fiber of the first embodiment, dispersion compensators of −35 +35ps/nmwere constituted from grating pairs, and the dispersion compensatorswere built into the optical repeaters, following which a transmissionexperiment similar to the first embodiment was conducted. In thisexperiment, a good result of approximately 1 dB was obtained for theamount of deterioration of reception sensitivity after transmission. Inthe present embodiment, optical fibers for dispersion compensation maybe mounted in the optical repeaters in place of the dispersioncompensators.

Next, the third embodiment of FIGS. 6(A) and 6(B) will be described.

An optical transmitter 3 drives the current to be supplied to asemiconductor laser source 1 with an electric signal of 5 Gbit/soutputted from a modulation signal source 5 and outputs a CPFSK(Continuous-Phase Frequency-Shift-Keying) optical signal modulationlight waveform. The CPFSK modulated optical signal is amplified to +6dBm by a first erbium-doped optical fiber amplifier 11 and outputted toa first transmission optical fiber 101. The transmission line opticalfiber 101 is an optical fiber of 100 km which has a normal dispersion(D<0) amount of −0.4 ps/km/nm and a loss of 21 dB at an oscillationwavelength of 1.552 μm of the semiconductor laser source 1. The signaltransmitted through the transmission line optical fiber 101 is againamplified to +6 dBm by a second erbium-doped optical fiber amplifier 12and outputted to a second transmission line optical fiber 111. Theoutput light of the optical fiber 111 is amplified by a third opticalamplifier repeater 13 and outputted to a third transmission line 121. Inthis manner, an optical amplifier lumped repeater system of 100 stageshaving a total distance of 10,000 km is constructed. In the opticalamplifier lumped repeater system, an optical fiber of 100 km of normal(D<0) dispersion similar to optical fiber 101 is employed fortransmission optical fibers 111, 121, . . .

An optical receiver 200 mixes the signal light that has passed the lastoptical transmission line 191 with the output of a local oscillationlight source 201 having a frequency that differs from that of thesemiconductor laser source 1 by 10 GH_(z) and detects the mixture signalby heterodyne detection by a PIN photodiode 51, which is a photoelectrictransducer. The heterodyne-detected signal is passed through a delaydetector 300 to reproduce it as an electric signal of 5 Gbit/s. Here, adelay equalizer 301 shown in FIG. 6(B) is not used.

The dispersion of the transmission optical fibers is not limited to −0.4ps/km/nm, and an optical fiber having a normal (D<0) or anomalous (D>0)dispersion region other than that value may be employed. It is to benoted, however, that taking the distribution of dispersion values in thelongitudinal direction of the optical fibers, it is effective to set thedispersion to a value in a somewhat excessively normal (D<0) dispersionregion in advance so that the zero dispersion of the optical fiber maynot occur at the signal light wavelength.

According to the above-mentioned experiments by Saito et al., whentransmission was performed with the signal wavelength set to coincidewith the zero dispersion wavelength, an error rate floor phenomenon wasobserved when the transmission distance is over approximately 2,500 km.However, when a transmission optical fiber was set to a normal (D<0)dispersion region as in the present invention, the noise-increasingeffect due to a non-linear effect was suppressed and no floor phenomenonwas observed. However, reception sensitivity was degraded byapproximately 7 to 8 dB due to the influence of the dispersion of thetransmission line, as indicated by an alternate long and short dashes inFIG. 7. Further, while some influence of self-amplitude modulationpeculiar to coherent communications was observed, no significantwaveform degradation was found because the dispersion of thetransmission optical fiber was set to a value in a normal (D<0)dispersion region and the dispersion value was low.

Further, it was attempted to compensate for the influence of dispersionof a transmission line upon a heterodyne-detected electric signal in anintermediate frequency band using a delay equalizer 301, as shown inFIG. 6(B). A conventional strip line circuit was used for dispersioncompensation. The amount of compensation of the strip line circuit wasset to 4,000 ps/nm so as to compensate for the total amount oftransmission line dispersion. By detecting the electric signal by delaydetection after the electric signal passed the delay equalizer, thesensitivity degradation amount was suppressed to below 3 dB, asindicated by a broken line in FIG. 7.

The present invention may be modified in numerous ways in addition tothose described above. For example, it is possible to set thetransmission wavelength to a value in a anomalous (D>0) dispersionwavelength band of a transmission optical fiber and employ a normal(D<0) dispersion optical fiber as the optical fiber for compensating forthe anomalous (D>0) dispersion or to use a normal (D<0) dispersionoptical fiber and a anomalous (D>0) dispersion optical fiber havingequal absolute dispersion values and equal distances. Further, thenumber of kinds of optical fibers used for each section is also notlimited to two but may be three or more. If the total amount ofdispersion for each section is set to a value in the proximity of zero,the lengths of anomalous (D>0) and normal (D<0) dispersion opticalfibers can be set freely for each section. Also, the number of repeatingstages is not limited to 10 stages, but may be more or less than 10stages, including for example 20 or 100 stages. Further, the length ofeach section may be greater or smaller than 100 km, including forexample 50 km or 150 km, and the bit rate used may also be higher orlower than 10 Gbit/s, including for example 2.5 Gbit/s, 5 Gbit/s or 20Gbit/s.

Further, the modulation system is not limited to intensity modulationbut may also be frequency modulation or phase modulation. Also, thereception system is not limited to a direct detection system, and aheterodyne detection system may be employed. In addition, the opticalamplifier for use with the optical amplifier lumped repeater system isnot limited to an erbium-doped optical fiber amplifier but may be asemiconductor laser amplifier, a praseodymium-doped (Pr+3) optical fiberamplifier or an optical Raman amplifier. Also, the wavelength band ofthe transmission light source is not limited to the 1.5 μm band, but the1.3 μm band may be used instead.

It is to be understood that variations and modifications of “OpticalCommunication Transmission System” disclosed herein will be evident toone skilled in the art. It is intended that all such modifications andvariations be included within the scope of the appended claims.

What is claimed is:
 1. A long-haul optical amplifier lumped repeater communication system comprising: an optical transmitter generating a light signal; a first optical transmission link connected to said optical transmitter and having a zero dispersion wavelength of a value different from the transmission wavelength of the optical transmitter; a plurality of optical amplifier-repeaters, one of the optical amplifier-repeaters being connected to the first optical link; a plurality of intermediate optical transmission links interconnecting the plurality of amplifier-repeaters, each of said plurality of optical transmission links having a zero dispersion wavelength of a value different from the transmission wavelength of the optical transmitter; an optical receiver; a terminating optical transmission link connected between one of the optical amplifier-repeaters and the optical receiver and having a zero dispersion wavelength of a value different from the transmission wavelength of the optical transmitter; a plurality of dispersion compensating means, each connected to respective one of said first, intermediate and terminating optical links so that the total dispersion of the system is approximately equal to zero.
 2. The long-haul optical amplifier lumped repeater communication system of claim 1, wherein said plurality of dispersion compensating means introduce chromatic dispersion so that the total dispersion of each optical transmission link and a dispersion compensating means connected thereto adds up to zero.
 3. The long-haul optical amplifier lumped repeater communication system of claim 1, wherein said optical transmitter includes an optical amplifier and wherein said optical receiver includes an optical amplifier.
 4. The long-haul optical amplifier lumped repeater communication system of claim 1, wherein said plurality of dispersion compensating means are included in said plurality of optical amplifier-repeaters.
 5. An optical amplifier repeater in an optical transmission line with an optical transmission link having a zero dispersion wavelength of a value different from the wavelength of a received optical signal for amplifying said received optical signal from said optical transmission link comprising: a dispersion compensator connected to said optical transmission link for compensating chromatic dispersion resulted from said optical transmission link such that chromatic dispersion substantially equals zero and outputting a compensated signal; an optical amplifier connected to said dispersion compensator for optically amplifying said compensated signal and outputting an amplified signal as an output optical signal of said optical amplifier repeater.
 6. An optical amplifier repeater as claimed in claim 5, wherein said dispersion compensator comprises an optical fiber having chromatic dispersion characteristics of different polarity from that of said optical transmission link.
 7. A long-haul optical amplifier lumped repeating method for transmitting an optical signal via first to N-th optical transmission links which are connected in a cascade arrangement, each of said first to N-th transmission links having a zero dispersion wavelength of a value different from the wavelength of said optical signal, where N is an integer greater than 1, comprising the steps of: (a) generating and supplying, at the transmitting side, an optical signal to said first optical link; (b) repeating, (N−1 ) times, the following step b- 1 ) from I=2 to I=N, b- 1 ) amplifying an optical signal supplied from said (I- 1 )th optical transmission link to output the amplified optical signal to said Ith optical transmission link: (c) compensating chromatic dispersion included in at least one of the output optical signals from said first to (N−1 )th optical transmission links; and (d) supplying an optical signal from Nth optical transmission link to a destination side, wherein at least one step of compensating for chromatic dispersion is inserted in said step (b), said compensating step compensating chromatic dispersion included in said optical signal supplied therein so that the total dispersion from the transmitting side to said destination side may be substantially equal to zero.
 8. An optical communication transmission system, comprising: a plurality of optical amplifiers located in a cascade arrangement on a transmission line having a zero dispersion wavelength at a wavelength different from the wavelength of a transmission light, for optically amplifying the transmission light to output an amplified transmission light; and at least one dispersion compensator located between said optical amplifiers, for compensating chromatic dispersion included in said amplified transmission light such that the chromatic dispersion included in said amplified transmission light from said transmission line substantially equals zero.
 9. An optical communication transmission system as claimed in claim 8, wherein said dispersion compensator makes the total dispersion of said system substantially equal to zero.
 10. An optical communication transmission system as claimed in claim 8, wherein said optical amplifiers include an erbium doped optical fiber amplifier.
 11. An optical communication transmission system as claimed in claim 8, further comprising: a light generator for generating said transmission light; and a dispersion shift optical fiber located between said optical amplifiers, having a zero dispersion wavelength of a value different from the wavelength of said transmission light.
 12. An optical communication transmission system as claimed in claim 11, wherein said light generator comprises: a semiconductor laser source for outputting optical power; a modulator for modulating said optical power with a modulation signal having a predetermined bit rate to generate said transmission light.
 13. An optical communication transmission system as claimed in claim 12, wherein said bit rate of said modulation signal is equal to or more than 10 GB/s.
 14. An optical communication transmission system as claimed in claim 12, wherein said bit rate of said modulation signal is equal to or more than 5 GB/s.
 15. An optical communication transmission system as claimed in claim 11, wherein said dispersion compensator comprises a dispersion compensation optical fiber having a zero dispersion wavelength of a value different from the wavelength of said transmission light.
 16. An optical communication transmission system as claimed in claim 15, wherein said dispersion compensation optical fiber is inserted after said dispersion shift optical fiber between said optical amplifiers adjacently located.
 17. An optical communication transmission system as claimed in claim 8, wherein the wavelength of said transmission light is selected to be either of a 1.3 μm band or a 1.5 μm band.
 18. An optical communication transmission system as claimed in claim 15, wherein said dispersion compensator comprises a dispersion compensation optical fiber having dispersion characteristics of different polarity from that of said dispersion shift optical fiber.
 19. An optical communication transmission system as claimed in claim 18, wherein said dispersion of said dispersion compensation optical fiber has a positive value and said dispersion of said dispersion shift optical fiber has a negative value.
 20. An optical communication transmission system as claimed in claim 19, wherein an absolute value of said dispersion of said dispersion compensation optical fiber is larger than that of said dispersion of said dispersion shift optical fiber.
 21. An optical communication transmission system as claimed in claim 15, wherein the length of said dispersion compensation optical fiber is shorter than that of said dispersion shift optical fiber.
 22. An optical communication transmission system having a plurality of optical amplifiers located in a cascade arrangement, each of said optical amplifiers amplifying transmission light to output an amplified transmission light, comprising: at least two pieces of dispersion compensation optical fiber having substantially a zero dispersion wavelength of a value different from the wavelength of said transmission light, for compensating dispersion included in said amplified transmission light; at least two pieces of dispersion shift optical fiber having substantially a zero dispersion wavelength of a value different from the wavelength of said transmission light; and wherein said dispersion compensation optical fiber and said dispersion shift optical fiber are alternately located with respect to one another and said plurality of optical amplifiers are connected to said interconnected dispersion shift optical fiber and dispersion compensation optical fiber; whereby the dispersion in the amplified transmission light substantially equals zero.
 23. An optical transmission signal transmitted on a transmission line having substantially a zero dispersion wavelength of a value different from the wavelength of said optical transmission signal, wherein said optical transmission signal is optically amplified into a first amplified optical transmission signal, dispersion included in said first amplified optical transmission signal is compensated such that the dispersion substantially equals zero, and said dispersion compensated first amplified optical transmission signal is optically amplified into a second amplified optical transmission signal.
 24. An optical transmission signal as claimed in claim 23, wherein said optical transmission signal is output from a semiconductor laser source.
 25. An optical communication transmission system, comprising: a plurality of optical amplifiers located on a transmission line constructed of a dispersion shift optical fiber having a zero dispersion wavelength of a value different from the wavelength of transmission light, for amplifying said transmission light; and a plurality of dispersion compensators located on said transmission line at a predetermined interval, for compensating dispersion included in said transmission light such that dispersion substantially equals zero.
 26. An optical communication transmission system as claimed in claim 25, wherein said dispersion compensator comprises a dispersion compensation optical fiber having substantially a zero dispersion wavelength of a value different from the wavelength of said transmission light.
 27. An optical communication transmission system as claimed in claim 25, wherein a wavelength of said transmission light is within a 1.5 μm band.
 28. An optical communication transmission method, comprising the steps of: generating transmission light; amplifying said transmission light by a plurality of amplifiers located in a cascade arrangement on a transmission line of dispersion shifted optical fibers having a zero dispersion wavelength of a value different from the wavelength of said transmission light to output amplified transmission light; and compensating dispersion included in said amplified transmission light between said optical amplifiers such that the dispersion of said amplified transmission light output from the transmission line is substantially zero.
 29. An optical amplifier repeater in an optical transmission line with an optical transmission link having a zero dispersion wavelength of a value different from the wavelength of a received optical signal for amplifying said received optical signal from the optical transmission link; comprising; a dispersion compensator for compensating chromatic dispersion resulted from said optical transmission link such that chromatic dispersion substantially equals zero and outputting a compensated signal; and an optical amplifier for optically amplifying said compensated signal and outputting an amplified signal as an output optical signal of said optical amplifier repeater. 